Various phosphorus containing compounds have already been investigated for their suitability as flame retardant additives. In particular, DOPO (9,10-dihydro-9-oxa-phosphaphenanthrene-10-oxide) and derivatives thereof have been explored as flame retardants and are known as predominantly active by a gas-phase flame retardant mechanism.
For example, it is known from EP 1506968 that DOPO-based phosphonates can be synthesized efficiently, whereas U.S. Pat. No. 4,228,064 demonstrates the utility of DOPO-based phosphonates and phosphinates as flame retardant additives for polyphenylene ether formulations. Additionally, EP 2557085 describes the synthesis of DOPO-based phosphonamidates and their application in polyurethane foams.
Furthermore, it is known from DE 10330774 and EP 2284208 that DOPO can be reacted with an unsaturated dicarboxylic acid and subsequently copolymerized with other P-containing derivatives, suitable to build ester bonds, to obtain flame retardant polyesters with varying P-content. These DOPO-based polymers can serve, for example as flame retardant additive for polyethylene terephthalate (PET) in fibre applications or as stand-alone flame retardant polyesters in engineering plastics.
Although the thermal stability of some of the reported DOPO derivatives would enable their melt processing at elevated temperatures as typically used for polyester processing, none of these derivatives were shown to form char upon pyrolysis, a property that would increase their efficacy as flame retardant additives.
Polymers and/or oligomers (no molecular weight is typically reported) containing pentaerythritol diphosphonate, a char former unit, having DOPO as a pendant group, such as reported by Wang X. et all in Materials Chemistry and Physics 2011, 125, 536-541 and the like, are known. However, for all the reported structures the question of having halogenated end-groups is not addressed. This might be an issue, in view of the fact that, for a relatively low molecular weight additive, the significance of the end-groups is increased.
Pentaerythritol phosphate alcohol (PEPA) is known as a flame retardant additive with relatively low thermal stability (decomposition around 200° C.), being active primarily in the condensed-phase, forming up to 40 wt % char upon pyrolysis. Derivatives of PEPA are mostly used as flame retardant additives for polyolefins, such as polypropylene, as described in U.S. Pat. No. 5,420,326.
U.S. Pat. No. 4,801,625 describes PEPA derivatives that in combination with other polymer additives, such as inert gas producing compounds (e.g. melamine) or other phosphorus containing derivatives (e.g. ammonium polyphosphate) render polyolefins flame retardant. WO 91/04295 teaches the use of PEPA derivatives to obtain smoke suppressed unsaturated polyester resin compositions.